Food product rigid containers generally must be impermeable to oxygen. Most common structural polymers for rigid food containers are permeable to oxygen which invades the food product causing degradation of spoilage. Those polymers which are sufficiently impermeable to oxygen generally are not suitable alone for rigid containers for foods because they do not possess adequate structural properties, are moisture sensitive, or are not approved for or are of questionable safety when used in contact with foods. Ethylene vinyl alcohol copolymer (EVOH) is a transparent extrusible material possessing high impermeability to oxygen when dry, many times less permeable than acrylonitrile copolymers, but is very moisture sensitive. The oxygen barrier properties of EVOH are markedly diminished in the presence of significant quantities of water. To be useful for food packaging, particularly where extended shelf life is required, EVOH must be kept dry as by total encapsulation within polymers which have good moisture barrier properties.
Many foods are processed in the container in a pressure cooker or retort. Retort conditions commonly are 250.degree. F. at 30 psia steam pressure. A rigid container must survive retort conditions. It must not permanently distort during cooking or during cooling, and must not suffer an alteration of the desirable properties of its components. Polyolefins, particularly blends or copolymers of polypropylene and polyethylene, are well suited to manufacture of rigid containers and have adequate physical properties to survive retorting. Polyolefins are relatively poor oxygen barriers, but are relatively good moisture barriers. The use of polyolefins with a central core of an oxygen barrier polymer is a desired goal of the food packaging industry.
Nohara et al. U.S. Pat. No. 3,882,259 discloses a three ply plastic bottle having a core of EVOH blended with Surlyn A brand ionomer resin and outer plies of polyethylene blended with Surlyn A. The Surlyn A ionomer is added to both the EVOH and the polyethylene resin materials to improve adhesion between layers. The bottle is to be made by extrusion blow molding thereby the three layers are simultaneously extruded to produce a three ply tube. While still hot from extrusion, the tube is pinched together at the bottom to form a seal and inflated in a blow mold having the shape of the desired bottle.
Extrusion blow molding has four serious drawbacks when used to form multi-layer containers having a core ply of a moisture sensitive barrier material such as EVOH.
First, the pinch seal at the bottom leaves the core ply of EVOH exposed on the bottle exterior. Since EVOH and certain other barrier materials are adversely affected by moisture, exposure of the core ply at the container bottom renders the container susceptible to loss of barrier quality by intrusion of moisture. The risk that the container exterior will encounter damp conditions in storage or transport is high and the resulting loss of barrier quality will degrade or spoil the food. First, retort conditions are such that moisture from the steam will intrude into the barrier layer through the exposed barrier at the bottom.
Second, extrusion blow molding necessarily produces scrap as a result of the pinch sealing procedure. Since the scrap contains materials from each of the three layers, re-extrusion of the scrap is difficult and expensive.
Third, the pinch seal produces a bottom of non-uniform thickness and strength. The sealing takes place along a line between the abutting faces of the inner layer material. The seal line is bordered by regions of relatively thick material. When stretched during blow molding, the bottom varies in thickness in the vicinity of the pinch seal. Because of the thickness variation due to the pinch seal, the stiffness of the bottom is not uniform along all diameters. Consequently, the bottom does not evenly respond to expansion and contraction as the product changes in temperature. This lack of even response causes unpredictable performance of the container when retorted.
Fourth, the pinch seal may create an interruption in the barrier layer. If the inside surface layer is interposed between the barrier layer at the seal, a line lacking barrier material will result. The area of the interruption may be great enough to allow sufficient oxygen to enter to be a problem.
Because of these disadvantages, extrusion blow molding cannot produce an entirely satisfactory three layer rigid container having a core barrier layer of a moisture sensitive polymer such as EVOH, particularly where the container is intended for retorting.